Aluminum Phosphide Poisoning

Aluminum phosphide is an indoor fumigant used to kill insects in most, if not all, of the stored grains we and our horses are exposed to. Like most pesticides, it can cause major problems in unintended species when used incorrectly. At the 2008 American Association of Equine Practitioners convention, held Dec. 6-10 in San Diego, Calif., a report of aluminum phosphide poisoning in 29 Texas horses was presented by Leslie Easterwood, DVM, a lecturer in veterinary medicine at Texas A&M University.

She explained that aluminum phosphide kills by causing rapid cell death and organ failure. It's commonly used as a suicide agent in India, she noted; it causes acute gastrointestinal distress, vomiting, and retrosternal burning (gastric reflux) in humans. (Sounds like a rough way to go!) There is no antidote.

In Texas, aluminum phosphide can only be used when a certified applicator is on the premises. It's typically left in the silo for several days (the exact time depends on the ambient temperatures), then it must be vented for at least 48 hours before the treated grains are fed. In this case series, about 400 pounds of grain in the silo was treated about 14 hours before it was fed to 66 horses, with no venting period.

"Twenty-nine horses showed clinical signs of profuse sweating (described as "a pool under each foot"), tachycardia (high heart rate), tachypnea (rapid breathing), fever, ataxia (incoordination), seizures, and widespread muscle tremors," Easterwood reported. She further explained that affected horses initially presented with one of two syndromes: Either they were severely affected when the veterinarians arrived and died within four hours, or their clinical signs were initially milder and worsened to recumbency (lying down with inability to rise) with seizures later in the evening. The latter cases' clinical signs tended to progress to more severe neurologic signs, including head pressing, circling, worsened ataxia, apparent blindness, and nonresponsiveness to stimuli. Only six horses were stable enough to transport to Texas A&M for intensive treatment.

Despite extensive supportive treatment at Texas A&M, four of those horses died. Five of the referred horses, including the two survivors, were treated with intravenous nutrition once it was realized that the horses were also hypoglycemic (had low blood sugar). The two survivors made a complete recovery. Ultimately, 27 of the 29 clinically affected horses died, most of them within half a day.

Extreme care should be taken to use aluminum phosphide only as directed to avoid such outcomes.